NMR study of low-energy excitations in Na β -alumina

²⁷Al and ²³Na NMR relaxation measurements are reported for Na β-alumina in the temperature range {~}4-120 K. Below {~}55 K, T₁ exhibits a power-law dependence on temperature similar to that observed in a wide variety of disordered solids and attributed to the presence of energetically low-lying excitations. The T₁ behavior in the temperature region {~}55-120 K can be described by a thermally activated motional process with EA≈0.05 eV. The ²³Na spin-spin relaxation time T₂ decreases monotonically with increasing temperature, suggesting that nuclear spin dephasing by ion diffusion in static electric-field-gradient inhomogeneities is appreciable.